This invention relates to power supplies.
The invention is more particularly concerned with power supplies for electric motors.
Brushless dc motors are driven by a commutation circuit which applies a drive voltage across the various stator coils in a switched sequence to produce a rotating magnetic field which causes the magnetic rotor to rotate. The torque/speed characteristic of a brushless dc motor is a function of: motor size, stator winding characteristics, load torque and applied voltage. For a given motor and load torque, the speed of the motor shaft is, therefore, proportional to the applied voltage.
Where a brushless dc motor is required to have a variable speed, it is common practice to connect it into a feedback loop and to use a pulse width modulation (PWM) amplifier to effect speed control. PWM amplifiers provide a variable level of output-drive voltage by varying the mark-space ratio of a square wave output. The switching between the ON and OFF states of the PWM amplifier is at a very much higher frequency than switching between different coils of the motor. The inductance of the stator coils smooths out the very high frequency square-wave voltage to a mean d.c. voltage level proportional to the mark-space ratio. PMW amplifiers have a high efficiency because their output drive circuit is always either in an OFF or an ON state. There is, therefore, a relatively small amount of power dissipated in the output drive circuit, all the power going to the motor.
The voltage applied to the motor is proportional both to the mark-space ratio of the PWM amplifier output supplied to the commutation circuit, and to the supply voltage applied to the motor via the commutation circuit. In many applications, the supply voltage can vary considerably with a consequent effect on the torque/speed of the motor. Although the supply voltage can be regulated by conventional means, the high load currents required by brushless d.c. motors means that series element power-supply regulators are inefficient, whereas switched-mode power supplies tend to be expensive and can be a source of electromagnetic interference.